It is generally known that a wheel bearing apparatus can support a vehicle wheel with respect to a suspension apparatus. Also, an incorporated rotational speed detecting apparatus detects a rotation speed of the vehicle wheel of vehicle to control the anti-lock braking system (ABS). Such a bearing apparatus generally includes a sealing apparatus arranged between the inner and outer members that rotate relative to each other via rolling elements contained between them. The sealing apparatus is integrally formed with a magnetic encoder, that has magnetic poles alternately arranged along its circumference. A rotational speed sensor detects changes of the magnetic poles of the magnetic encoder caused by the rotation of the wheel. The rotational speed sensor is adapted to be mounted on a knuckle, forming part of a suspension apparatus, after the wheel bearing apparatus has been mounted on the knuckle.
A known structure is shown in FIG. 18 as one example of a wheel bearing apparatus. This wheel bearing apparatus has an outer member 50, an inner member 51, and a plurality of balls 52 contained between the outer member 50 and the inner member 51. The inner member 51 includes a wheel hub 53 and an inner ring 54 fit onto the wheel hub 53.
The outer member 50 has, on its outer circumference, an integrally formed body mounting flange 50b. The outer member inner circumference includes double row outer raceway surfaces 50a, 50a. A sensor 63 is secured on the knuckle 65 via a bolt 66.
The wheel hub 53 is integrally formed with a wheel mounting flange 55 to mount a wheel (not shown) on one end. The wheel hub 53 has an inner raceway surface 53a and a cylindrical portion 53b axially extending from the inner raceway surface 53a. An inner ring 54, with an inner raceway surface 54a formed on its outer circumference, is axially immovably secured to the wheel hub 53 by a caulked portion 53c formed by plastically deforming the end of the cylindrical portion 53b. 
A sealing ring 56 is fit into the outer end of the outer member 50. A lip of the sealing ring 56 slidably contacts a base portion 55a of the wheel mounting flange 55. On the other hand, an encoder 57 is mounted on the inner end outer circumference of the inner ring 54. The encoder 57 has an annular supporting member 58, with an L-shaped longitudinal section, and a ring-shaped encoder body 59 adhered to the side of the annular supporting member 58 along its entire periphery. The encoder body 59 has N and S poles alternately arranged along its circumference.
The inner end opening of the outer member 50 is closed by a cover 60. The cover 60 is formed of non-magnetic sheet material such as non-magnetic stainless steel sheet, aluminum alloy sheet or high functional polymer, etc. It has a dish-shaped configuration with a disk-shaped closing plate portion 61 and a cylindrical fitting portion 62 formed around a periphery of the closing plate portion 61.
The side face of the encoder body 59, forming the encoder 57, is arranged opposite and close to the cover 60. The detecting portion 64 of the sensor 63 is arranged close to or abutting against the side of the cover 60. Thus, the detecting portion 64 is arranged opposite and close to the encoder body 59 via the cover 60. Accordingly, the presence of the cover 60 prevents the entry of water, iron powder or magnetized debris, etc. into the space between the sensor 63 and the encoder 57. Thus, this prevents damage to the sensor 63 and the encoder 57, as well as cyclic interfere or deterioration of the magnetic properties of the encoder body 59 (see, e.g. JP2000-249138A and JP2007-1341A).
Low cost production and reduction of weight and size for fuel saving consumption of the wheel bearing apparatus has been promoted. In such a wheel bearing apparatus, a wheel hub and a double row rolling bearing are united, a representative example is shown in FIG. 20.
This wheel bearing apparatus has an outer member 70, a pair of inner rings 71, 71, and a plurality of balls 72 contained between the outer member 70 and the inner rings 71, 71. The outer member 70 has, on its outer circumference, an integrally formed wheel mounting flange 73. Its inner circumference has double row outer raceway surfaces 70a, 70a. 
The pair of inner rings 71, 71 are fit onto an axle shaft 74 via a clearance fitting and axially secured by a securing nut 75. A shield 76 and a seal 77 are mounted in annular openings formed between the outer member 70 and the inner rings 71, 71. They prevent leakage of lubricating grease sealed within the bearing and the entry of rain water or dust from the outside into the bearing. The shield 76 forms a labyrinth seal cooperating with the inner ring 71 to prevent the flowing out of lubricating grease.
The outer member 70 is formed with a brake pilot 78 and a wheel pilot 79 axially extending from the wheel mounting flange 73. The brake pilot 78 is formed as a cylindrical projection coaxial with the outer member 70. The wheel pilot 79 is formed with a diameter slightly smaller than that of the brake pilot 78 and split circumferentially into several parts.
A cap 80, formed of steel sheet having corrosion resistance, is press worked into a cup-shaped configuration. As shown in FIG. 21(a), the cap 80 has a cylindrical portion 80a, an overlapped portion 80b, and a bottom portion 80c. The overlapped portion 80b projects radially outward from the cylindrical portion 80a. The bottom portion 80c continuously extends from the overlapped portion 80b. 
In addition, the cap 80 is formed with a plurality of claws 81 along the circumference of the cylindrical portion 80a. Each claw 81 is raked radially outward by a slight angle. An annular groove 82 is formed on an inner circumference of the brake pilot 78, as shown in FIG. 22. The cap 80 can be securely fitted onto the outer member 70 by press-fitting the cap 80 into the opening of the outer member 70 until the overlapped portion 80b abuts against the end face of the pilot 79, due to spring-back engagement of claws 81 with the annular groove 82. Thus, the circumferential inner surface of the pilots 78, 79 can be prevented from corrosion (see, JP 2000-249138A and JP 2007-1341A).
However, problems exist in these wheel bearing apparatus of the prior art. First of all, in the former wheel bearing apparatus, since the cover 60 is secured on the outer member 50, via metal-to-metal fitting, it is impossible to have sufficient sealability in the fitting portion without improving the accuracy and roughness of the fitting surfaces.
In addition, since the cover 60 has a simple “C”-shaped longitudinal section, the rigidity of the cover 60 is insufficient. Thus, the cover 60 is deformed by impingement of pebbles and is caused to contact with the encoder body 59.
Furthermore, since the detecting portion 64 of the sensor 63 opposes the encoder 57 via the cover 60, it is necessary to take into consideration not only the sheet thickness of the cover 60 but air gaps between the encoder 57 and the cover 60 as well as the cover 60 and the sensor 63. This causes an increase of the air gap between the encoder 57 and the sensor 63 and thus reduction of the sensitivity of the sensor 63 and the detection accuracy.
In addition, the cover closing plate portion 61 sometimes deforms by the press-fitting interference between the cover 60 and the outer member 50, as shown by two dotted lines in FIG. 19. In such a case, it is necessary to previously consider the amount of deformation of the cover 60 and to set a large initial air gap in order to avoid interference between associated parts.
In the latter wheel bearing apparatus of the prior art, with the split-type pilot portions 78, 79, it is possible to prevent the inside of the bearing from getting rusty as well as to inhibit the rust invasion into the pilot portions 78, 79, by securely fixing the cap 80 onto the outer member 70. The cap 80 is press fit into the split-type pilot portions 78, 79. The cap 80 is formed of steel sheet by press-working. Thus, it is difficult to have high accuracy dimensions and roundness of the cylindrical portion 80a. Accordingly, the interference of the cylindrical portion 80a is usually set large such as a range within 0.1˜0.4 mm to prevent movement and slip-off of the cap 80.
Furthermore, if a width “A” (FIG. 22) of a guiding portion 83 of the pilot portions 78, 79 is insufficient in addition to the large interference of the cylindrical portion 80a, the cap 80 can sometimes be press-fit into the pilot portions 78, 79 under an inclined condition. This causes linear damage or scratches 84 (FIG. 21(b)) on the cylindrical portion 80a of the cap 80 during press-fitting. Thus, it is believed that sealability of the cap 80 is detracted by the damage or scratches 84 and accordingly the life of the wheel bearing apparatus would be reduced by generation of corrosion due to the entry of rain or muddy water through the fitting portion.